1. Field of the Invention
This invention relates to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56 (1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image of the object stored thereon, and is then scanned with stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion t the stored radiation energy. The light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays is photoelectrically detected and converted into an electric image signal, which is processed to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal by use of a photoelectric conversion means to reproduce a visible image on a recording medium such as a photographic film or a display device such as a CRT.
In the aforesaid radiation image recording and reproducing system, read-out of the radiation image is generally conducted by use of a read-out apparatus constituted so that a beam of stimulating rays deflected by a light deflector is made to scan on the stimulable phosphor sheet in a main scanning direction, and at the same time the stimulable phosphor sheet is conveyed in a sub-scanning direction approximately normal to the main scanning direction.
As the light deflector, a multi-face rotating mirror (i.e. polygon mirror) rotating at a high speed may be used. The multi-face rotating mirror is advantageous in scanning stability over other light deflectors such as a galvanometer mirror. However, for achieving high scanning stability, it is necessary to rotate the multi-face rotating mirror at a high speed. On the other hand, in order to stimulate the stimulable phosphor sheet carrying a radiation image stored thereon, it is necessary to expose the sheet to stimulating rays of comparatively high energy. However, when the multi-face rotating mirror is rotated at a high speed, the scanning speed of the beam of stimulating rays in the main scanning direction becomes high, and the level of stimulation energy which the stimulable phosphor sheet receives decreases. As a result, the level of light emitted by the stimulable phosphor sheet decreases, and the S/N ratio of a read-out image signal becomes low.